# num = int(input("输入整数："))
# i = 0
#
# while i <= num:
#     if num ==0:
#         break
#     print("*"*num)
#     num = num- 1

# aStr = input("输入文字：")
# i = 0
# for a in  aStr:
#     if a == " ":
#         i = i+1
# print(i)

# for i in range(101):
#     c = (i*(i+1))%11
#     if c == 8:
#         print(i)
#
# i =0
# for x in range(5):
#      for y in range(10):
#          print('hello')
#          i=i+1
#          print(i)

# a = input("输入一个整数：")
# a = int(a)
#
# for i in range(a):
#     for j in range(a):
#         print(j+1,end=" ")
#     print()
#
# for a in range(1,101):
#     if a%2==0:
#         continue
#     print(a)
i = int(input("整数判断："))

# for a in range(i):
#     if a==1 or a==0:
#         continue
#     if i%(a)==0:
#         print("这个不是是字数")
#         break
#     else:
#         if i ==a+1:
#             print("shi")
num = 1
sn =0
for a in range(1, i, 2):
    c = 1/a
    if num%2==0:
        sn=sn-c
    else:
        sn=sn+c
    num = num +1
print(sn*4)